Intel IA-32 - Manual (Page 560)

15-10 Vol. 3A
8086 EMULATION
The 8086 operating-system services consists of a kernel and/or operating-system procedures
that the 8086 program makes calls to. These services can be implemented in either of the
following two ways:
•
They can be included in the 8086 program. This approach is desirable for either of the
following reasons:
— The 8086 program code modifies the 8086 operating-system services.
— There is not sufficient development time to merge the 8086 operating-system services
into main operating system or executive.
•
They can be implemented or emulated in the virtual-8086 monitor. This approach is
desirable for any of the following reasons:
— The 8086 operating-system procedures can be more easily coordinated among several
virtual-8086 tasks.
— Memory can be saved by not duplicating 8086 operating-system procedure code for
several virtual-8086 tasks.
— The 8086 operating-system procedures can be easily emulated by calls to the main
operating system or executive.
The approach chosen for implementing the 8086 operating-system services may result in
different virtual-8086-mode tasks using different 8086 operating-system services.
15.2.3 Paging of Virtual-8086 Tasks
Even though a program running in virtual-8086 mode can use only 20-bit linear addresses, the
processor converts these addresses into 32-bit linear addresses before mapping them to the phys-
ical address space. If paging is being used, the 8086 address space for a program running in
virtual-8086 mode can be paged and located in a set of pages in physical address space. If paging
is used, it is transparent to the program running in virtual-8086 mode just as it is for any task
running on the processor.
Paging is not necessary for a single virtual-8086-mode task, but paging is useful or necessary in
the following situations:
•
When running multiple virtual-8086-mode tasks. Here, paging allows the lower 1 MByte
of the linear address space for each virtual-8086-mode task to be mapped to a different
physical address location.
•
When emulating the 8086 address-wraparound that occurs at 1 MByte. When using 8086-
style address translation, it is possible to specify addresses larger than 1 MByte. These
addresses automatically wraparound in the Intel 8086 processor (see Section 15.1.1,
“Address Translation in Real-Address Mode”). If any 8086 programs depend on address
wraparound, the same effect can be achieved in a virtual-8086-mode task by mapping the
linear addresses between 100000H and 110000H and linear addresses between 0 and
10000H to the same physical addresses.